Start-stop oscillators are particularly useful in applications where it is desired to provide a burst of oscillations which begin in a predictable phase with respect to a control or keying signal. As an example, such an oscillator may be used to advantage for providing a character dot clock signal to a character generator in a television receiver. By keying the oscillator in synchronism with the scanning of the raster, the character dots produced by the character generator will occur at predictable scan times and the displayed caption formed by the character dots will thus be stable with respect to the displayed picture image. There are, of course, many other uses for start-stop oscillators.
It is known to employ an inductor-capacitor (L-C) resonator as the frequency determining element in a start-stop oscillator. Such "tank" circuits provide excellent frequency stability but are difficult to operate in a pulsed mode because of the energy storage characteristics of the reactive elements. Prior art solutions to this problem have not been entirely satisfactory. As an example, in U.S. Pat. No. 3,991,388 there is described a start-stop oscillator in which oscillations are quickly damped by means of a pair of transistor switches. The circuit includes an inductor connected in parallel with a pair of serially connected capacitors to form the tank circuit of a Colpitts oscillator. The switches, when closed, provide a path to ground for each of the three elements of the tank circuit so that the energy stored in each element is quickly dissipated, thereby rapidly stopping the oscillator. The disadvantage of such an arrangement is that dissipation of the tank circuit energy to stop the oscillator results in a prolonged startup time since the energy must be replaced to initiate oscillation.
It is known that one may initiate oscillations in a resonator circuit quickly and with a predictable starting phase by passing a direct current through the inductor to cause energy to be stored therein and then interrupting the current by means of a switch to thereby release the stored inductor energy which then oscillates between the inductor and capacitors of the resonator circuit. An example of a start-stop Colpitts oscillator employing this principle is described by Gercekci et al. in U.S. Pat. No. 4,272,736. The Gerekci et al. oscillator is proposed for use as the "dot clock" for a character generator and comprises a resonator coupled between input and output terminals of an N-channel field-effect transistor (FET) amplifier. A pair of N-channel transistor switches are coupled between each end of the resonator and respective positive and ground supply terminals. During the period that the oscillator is turned off (e.g. horizontal blanking) a control circuit turns the transistor switches on to apply DC bias to the resonator and concurrently removes gate bias from the amplifier load transistor to disable the amplifier. The oscillator is started by turning the transistor switches off and restoring gate bias to the amplifier load transistor. Since energy is stored in the resonator as a consequence of the DC bias applied while the oscillator was off, the oscillator starts immediately and with a fixed starting phase.